Advanced control systems are becoming commonplace in the processing industry. A properly designed advanced control system can greatly increase the efficiency of a processing unit. With their proven ability to save money, we can only expect to see them in greater use in the future. However, like many other new technologies, advanced control is not without its share of problems.
Many of the problems that tend to arise with advanced control are not specifically hardware related. The problems are interface problems, or how the displays, system messages, and system alarms are configured and presented to the operators. For all of its virtues, there are several human factors problems associated with advanced control systems of which designers and users should be aware.
One of the human factors problems of advanced control is skill degradation. A primary cause of skill degradation is forgetting. Forgetting occurs due to the natural decay, over time, of the person's memory. Without practice, all memories will decay some amount and forgetting occurs.
Skill degradation can occur when a process is converted from pneumatic to DCS, and then to advanced control because the amount of “practice” the person has with the control systems is reduced. In one study conducted by Beville Engineering, control operators using DCS made 40% fewer control moves than operators using pneumatic control systems. The study also found that when compared to a similar process without advanced control, process units that have advanced control cut in half the number of control moves the operator makes per hour.
Humans can be thought of as adaptive control elements. People can “adapt” to the dynamics of a control system through practice. For example, a novice driver’s first experience behind the wheel of a car can be a terrifying experience. It will take the student several hours of practice to learn the dynamics of the car’s control systems (e.g., how much force is necessary to apply to the brakes, steering, and gas pedal in order to drive the car smoothly). Over time, without practice, the person's memory of the dynamics of the control system degrades, and the person will have to relearn the dynamics of the system. You have experienced this when you try to apply the brakes in your spouse's car after having not driven the car in a while.
In reducing the control requirements, operators do not practice putting inputs into the systems and seeing the results, and they will “forget” the system’s dynamics for that portion of the process that is on advanced control. For example, furnaces are a common application for advanced control. If the operator has had the furnace on advanced control for a while and must return to manual control, the operator will likely overshoot every time he tries to make an adjustment to the system until he has relearned the dynamics of the system.
Skill degradation and forgetting can also lead to another problem, manual takeover during process upsets and emergencies. What will happen during a process upset when the process is already bouncing around, and the operator whose skills have degraded must take manual control and stabilize the process? If you said it will take the operator longer to stabilize the system, you were right. It is for this reason that, if the operator must retake control of a system that has been on advanced control, he needs to have had some means to practice making control changes with that portion of the system prior to taking control.
Another problem with advanced control is that the displays are sometimes configured in a manner that makes it difficult for the operator to determine exactly what the control system is doing (i.e., the dynamics). If advanced control displays are designed in a manner that precludes the operator from understanding what exactly the control system is doing, the operator will quickly lose trust in the system and not use it. This is a common occurrence in the airline industry with autopilots. It is not uncommon for pilots to disengage autopilots when they can not figure out why the autopilot made what seemed like an unnecessary control change.
Delay in diagnosing system failures is another problem that many operators have experienced with advanced control. The delayed diagnoses create a delay in problem corrections. This happens when a failure occurs and the advanced control system begins to compensate for the problem. The advanced control will keep trying to compensate for the failure until it has reached a parameter limitation and turns off. Once the advanced control system has shut off, the operator must look back through the data and try to reconstruct what the advanced control system was doing, and deduce the original failure. Obviously, having poorly designed display systems further delays the diagnosis and correction of the problem.
When designing advanced control systems, being aware of some of the human factors problems can help the designer avoid the problem causes. One key to a well designed advanced control system is properly configuring the display and alarm systems in accordance with human factors principles.
Copyright © 1994 Beville Engineering, Inc., All Rights Reserved
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